Prior motion detection mechanisms used for indicating the speed of airborne objects have typically been costly, complicated, or bulky when effective at all. Many current aeronautical applications, such as missiles, short take-off and landing (STOL) aircraft and remotely piloted vehicles, require sensitive, reliable, and accurate motion detectors of low weight and volume and have thus created needs unsatisfied by prior art devices.
U.S. Pat. No. 3,241,374, for example, which provides for the sensing of heat, or convection, currents in a sensor attached to a moving body in order to determine acceleration of the body, has lacked accuracy and sensitivity where thermal changes and thermistor variations can adversely affect performance. In addition, the size and complexity of the prior art unit has made it impractical for the aforementioned applications.
Other references, such as U.S. Pat. No. 3,106,678, have avoided temperature effects by using various electrical or electronic characteristics in measuring the motion of a body. For example, as in the cited reference, bunches of positive ions are formed at an electrode and are then accelerated electrically through cathodes toward "vanes" which sense the ions as they pass. By comparing and mixing the frequencies of the ion signal as it passes different points along the path of flow of the ions, the acceleration of the body is determined. The positive ion generator used in conjunction with various types of electrodes, converters, and phase comparators, however, has diminished the ruggedness and simplicity of the electronic motion detector. Further, the prior art discloses closed-loop operation and the requirement of propelling the ions electrically even during periods when the body is stationary. Finally, some of the prior art techniques are also complicated by features which require the counting of ions or the mixing of ion signals or the deflection of an ion beam in response to the acceleration of the body. Prior art apparatuses have thus generally required considerable, sensitive, costly circuitry to perform their desired function.